#include <iostream>
using namespace std;

class C1
{
  int m_A; // 默认权限是私有
};

struct C2
{
  int m_A; // 默认权限是公有
};

int main(int argc, char const *argv[])
{
  C1 c1;
  // c1.m_A = 100;

  C2 c2;
  c2.m_A = 100;

  return 0;
}